Purpose:

Axial USpA is frequently considered to be an early form of ankylosing spondylitis (AS). Identifying the most highly differentially expressed PBMC gene will be useful in generating new diagnostic test and new experimental approach to pathogenesis of USpA and AS.

Method:

Genome-wide microarray analyses followed by Realtime PCR validation of 25 promising candidates were carried out on PBMC from 20 healthy subjects, 21 AS and 28 axial USpA patients. 11 of the validated candidates were assessed again by PCR using a second cohort of 18 USpA, 23 AS, 12 RA, 8 mechanical low back pain patients and 26 healthy control subjects.

Results:

Both microarray and PCR assays of the first cohort showed that the number of differentially expressed genes in USpA was > 6X more numerous compared to AS, which was surprisingly much less different from healthy subjects. Proinflammatory IL-1A, IL-1B, IL-6, IL-8 and several chemokines were differentially expressed only in USpA but not in AS. PCR results of the second cohort were in agreement. In both cohorts, RGS1 (regular of G-protein signaling-1) was identified as the most outstanding being 20.9 and 11.1 fold higher in USpA and AS compared to healthy subjects (p=0.000004 and 0.0002 after Bonferroni correction). RGS1 was not enhanced in RA or in those with mechanical low back pain. For distinguishing the combined USpA and AS group, the diagnostic potential was: sensitivity 84.2%, specificity 87.5%, +LR 6.7, PPV 97, NPV53.8. Evaluation of the specificity of RGS1 in the RGS family was carried out by comparing it to other members of the RGS family. PCR of the following common RGS members showed that only RGS1 carried diagnostic value: RGS -2, -3, -4, -5, -8. Next, to identify the responsible variant, we tested for RGS1 using 2 other pairs of primers spanning 2 different segments of the gene. The diagnostic potential was the same with all 3 pairs of primers. They also showed that the RGS1 isoform which was responsible was the Sp1 isoform. Although RGS1 has already been identified as candidate gene for diabetes and celiac disease, how it might cause pathology is unclear. Here, we tested 25 potentially arthritis-causing cytokines/chemokines with a cell line. TNFa and IL17 were discovered to be the strongest inducers of RGS1.

Conclusion:

(1) The PBMC of USpA is different from AS in having many more highly expressed genes, including several which are proinflammatory. (2) The TNFa- and IL-17- inducible RGS1 is the most highly differentially expressed gene in USpA. (3) The role of RGS1 in SpA and other RGS1-related diseases is a new area of research.